One process for the preparation of phosphoric acid is known as the "wet process". This process involves the acidulation of phosphate rock with a mineral acid such as sulfuric, phosphoric or nitric to produce the phosphoric acid. During acidulation of the rock there will be formed, in addition to the phosphoric acid, a solid precipitate. If sulfuric acid is the digesting acid the precipitate will include gypsum as its major constituent. In the conventional dihydrate process the aqueous phosphoric acid may be separated from the precipitate by filtration to produce a dilute or unconcentrated filter grade phosphoric acid product usually containing from 25-33% P.sub.2 O.sub.5. The resulting filter cake is countercurrently washed and eventually discharged. The filtrate from the cake washings are used as future cake washing solutions or are returned to the reaction section.
Phosphate rock contains various amounts of impurities and the level of impurities found in many remaining phosphate deposits is increasing. Among the increasing impurities are magnesium and/or aluminum containing compounds. When the phosphate rock is digested with sulfuric acid, as discussed above, the metal compounds (usually salts and oxides of the metal) may readily go into solution. These impurities may then form precipitates and settle out as solids or sludge over a period of many days or even months. The filtration rate of liquids containing dissolved metal ions and finely divided solids is low and the filter media is easily clogged. The metal ion containing impurities are difficult and costly to remove from the phosphoric acid because when precipitated they may carry with them very appreciable quantities of valuable P.sub.2 O.sub.5 which is so combined that it cannot be effectively removed by leaching with water or dilute acid.
Further, when the phosphoric acid (25-33% P.sub.2 O.sub.5) is concentrated the solubility of many of the initially soluble impurities may be exceeded and further precipitation occurs. Also problems may exist when the phosphoric acid is neutralized, or otherwise treated, as for example in the manufacture of liquid plant food, wherein wet process phosphoric acid and ammonia are reacted to produce a substantially neutral solution for use as a liquid plant food base. During the neutralization of the acid which contains the dissolved impurities the precipitation of sludge, especially magnesium containing salts, can occur. These impurities act to form deposits which may clog the transfer and distribution equipment. These impurities also result in loss of P.sub.2 O.sub.5 values and these sludges may hinder the removal of other types of impurities.
Various methods have been suggested in the prior art to obtain a more pure form of phosphoric acid. According to U.S. Pat. No. 3,935,298 iron impurities can be removed by mixing an alkali metal halide with the starting phosphate rock and firing the mixture at a temperature of 300 to 1100.degree. F. of before acidulation. U.S. Pat. No. 3,562,769 teaches the use of alkali metal salts as disintegration-preventing agents which can be used to retain impurities in a solid state while phosphoric acid is extracted, while U.S. Pat. No. 3,408,162 discloses that by adding a relatively small amount of an alkaline salt of lignosulfonic acid to the wet process phosphoric acid solid impurities may be maintained in suspension and crystal growth may be inhibited. U.S. Pat. No. 3,554,694 discloses use of a sodium salt addition in order to remove the fluorine present and obtain a commercially pure sodium fluosilicate.
Other methods exist for the removal of aluminum. U.S. Pat. No. 3,843,767 shows treatment of the original ore with pure phosphoric acid to obtain a low concentration of aluminum and iron impurities. U.S. Pat. No. 2,954,287 shows the use of sulfuric acid in combination with alkali salts in order to keep free aluminum concentration low. In another process, shown in U.S. Pat. No. 2,494,736, silicon is first removed and then fluorine and sodium are added to form a crystalline sodim/aluminum/fluorine salt that is easily separable. One other process deals with the recovery of fluorine from phosphate ore which involves precipitating aluminum and fluorine in the form of fluoaluminate. See, for details, U.S. Pat. No. 3,512,927.
German Patent Application No. 2,046,295, based on U.S. patent application Ser. No. 866,752, recommends the addition of a source of soluble fluoride to wet process acid to precipitate magnesium impurities as magnesium-aluminum-fluoride salts. It proposes that the precipitation be induced in concentrated (45-53% P.sub.2 O.sub.5) wet process acid to avoid competition between silicon and aluminum for the added fluoride ion. However, the high viscosity of concentrated wet process acid makes phase separation difficult. Further, the process recommends adjustment of the aluminum to magnesium mole ratio to at least 1.11 and preferably 2.37 to 9.48. The fluoride to magnesium mole ratio is recommended to be at least 4.67 and preferably between 6.37 and 25.5.
It is believed that the foregoing processes have not obtained a high efficiency and that they tend to be heavily burdened with capital expense necessary for production equipment. The techniques involving precipitation of impurities have advantages over other potential purification techniques due to the simplicity and compatibility with existing phosphoric acid facilities. However, precipitated impurities within the acid can settle out to damage equipment and clog filters and they are responsible for a substantial loss of P.sub.2 O.sub.5 values.
It is therefore desired to obtain better quality phosphoric acid, and it is also desired to be able to remove the impurities in as large a quantity and as early in the process as is possible, without encountering substantial processing problems.